NASA estimates that 45 percent of all first-day spacecraft failures and malfunctions are attributed to damage caused by high dynamic environments. While the study is over twenty years old, little has changed, as the problem is still present today. Spacecraft are subjected to a broad range of potentially damaging environmental conditions and stresses during flight, including shock.
At the US Space Foundation's 16th Annual National Space Symposium, a study counts a total of 2,147 payloads proposed for launch to Earth orbit during the next 10 years. The next wave of satellite launches will come in 2004-2006, led by new high-speed broadband multimedia communications satellites such as Motorola's Celestri, Boeing Satellite Systems, Alcatel Espace/Loral's SkyBridge/CyberStar, and Teledesic's Teledesic. It is estimated that there will be over 2,000 spacecraft launched in the next 10 years.
One of the highest shock events to the payload occurs at payload fairing separation. Fairings are attached to the booster at discrete attach points near the spacecraft interface by attachment bolts. The nuts and bolts must be strong enough to hold things in place until liftoff, yet when its time for parts to separate, they must release immediately. A pyrotechnic event or, in other words, an explosion, initiates the separation of the attachment nuts and bolts. These nuts and bolts are designed for use as standard high-strength attachment hardware, but possess the capability to detach components or structures on command.
A prior art device, shown in FIGS. 1 and 2, consists of a right hand threaded explosive nut 214 attached to a first space article and a left-hand threaded second explosive nut 218 attached to a second space article. A threaded stud 222 with right hand thread on one end and left handed thread on the other end interconnects the two nuts 214 and 218. The attached space articles, are pulled towards each other resulting in a preload on the system when the stud 222 is tightened into explosive nuts 214 and 218. The stud 222 is tightened to eliminate any gapping between the first space article and the second space article due to the high loads produced during the dynamic launch environment. A calibrated strain gauge, mounted onto the stud, measures the preload during the clamping process. The strain gauge installation and clamping process must be inspected and monitored to insure quality control. The preload on the 1.25-inch diameter stud is 70,000 lbs.
The explosive nuts 214 and 218 have an inner, internally threaded sleeve mechanism 226 and an outer retaining sleeve mechanism 224. The outer retaining sleeve mechanism 224 holds the inner internally threaded sleeve mechanism 226 at the proper diameter to receive the threaded stud. The separation process begins with a computer-controlled explosion of an ordnance charge 230 in the explosive nut 214 and 218. Computers handle the split-second timing of the detonation of each nut 214 and 218. The explosion drives the sliding of the inner sleeve mechanism 226 axially out of its retained position within the outer sleeve mechanism 224, splitting its threads and releasing its grip on the stud, separating it from the nut. The sudden release of the preload energy sends the nut 214 and 218 and stud 222 in opposite directions resulting in a sudden shock running through the first and the second space articles.
Modern spacecraft have more highly sensitive components than in the past, and must be designed to sustain high flight shock and vibration environments, at the expense of useful payload-to-orbit mass. High flight shock and vibration environments necessitate expensive time-consuming ground-tests to validate payload capabilities against the severe flight environment. These ground tests include random vibration tests, acoustics tests, and shock tests.
The traditional approach to shock protection of spacecraft places metal shock attenuation rings between the fairing and the payload attach fitting to dissipate the energy of the separation event and limit fairing separation shock to the payload. These rings are installed at the base of the fairing and, maintain contact between the launch vehicle and payload attachment hardware. The shock transmission path is “softened” by stacking the rings. However, shock rings have limited effectiveness as they still maintain contact between the payload adapter and the hardware. Current best-practice uses metal shock rings between the fairing and the payload attach fitting to dissipate energy of the separation event and limit fairing separation shock to the payload. Honeycomb crush blocks are also used to dissipate the shock of the explosive bolt.
Larger payloads will require larger fairings that in turn require stronger encapsulation joints in flight and demand a larger explosive charge for fairing separation. This trend produces fairing separation systems that will deliver even more shock to the payload. Large payload fairings incorporate up to three shock rings stacked between the fairing and payload adapter to mitigate payload shock to specification requirements. Testing results indicate that adding additional shock rings would yield diminishing payload shock mitigation. The separation sequence may fire 3 of the explosive bolts at a time rather than all of the bolts at once to further reduce the incidental shock. Clearly, new methods are required for improved payload shock protection.
Explosive attachment bolts add a great deal of monitoring complexity. Each bolt must be strain gauged and tightened by the requisite amount to remove gapping. The strain gauge calibration installation, inspections and monitoring during bolt tightening adds costly complexity to the launch vehicle.
Spacecraft are typically ground-tested to detect failures using random vibration, acoustic, and shock testing to simulate the launch environment. Typically, as shock survivability increases payload weight increases.
The elimination of the fairing separation shock event renders the design of payload accommodations benign from a dynamic environment perspective. The reduced shock makes the launch vehicle more desirable resulting in greater demand for missions. It would also eliminate the costly instrumentation, calibration, and quality control inspections required in the prior art explosive nuts. It also reduces part count, launch weight, cost and complexity.
The significant reduction of induced shock due to fairing and payload attach fitting separation events results in substantial cost savings for spacecraft manufacturers by eliminating the need to design a launch vehicle to survive these events. Spacecraft manufacturers will soon be writing low shock requirements into launch vehicle contracts to take full credit for cost and performance benefits. There is a need for a device that removes fairing separation shock as a performance-driver for spacecraft design. Multiple arrays, of this new device, could be fired simultaneously increasing reliability and reducing the cost and complexity of the launch vehicle, the avionics, the power supply, and the electrical routing.